Intricate excavation upgrades London metro station Mar 2021

Fig 1. Plan (left) and 3D image of the additional elements of the Bank Underground Station upgrade

Fig 2. Main working site and access shaft at Arthur Street

In a successful collaboration between Dr Sauer & Partners, Dragados and the London Underground project management team, the upgrade project of the Bank Underground Station in London began with award of the £642 million design-build contract in July 2013 and has progressed to substantial completion of the civil works in July 2020. With the aim of boosting capacity of the underground station by 40%, the new structures include:

• A new Northern line platform and running tunnel to convert the existing centre-island platform to separate platform tunnels;
• An additional entrance on Cannon Street;
• Step-free access to the Northern line platform level and improved access to the DLR (Docklands Light Rail) platforms; and
• A moving walkway between the Central and Northern lines

To complete the complex SCL (sprayed concrete lining) works, several technical achievements to improve constructability and protect the workers were developed and implemented successfully. These have included:

• Uphill and downhill excavation of 30 degree inclined escalator barrels in a top heading and combined bench/invert sequence;
• Removal of load bearing foundation piles of four high-profile stakeholder buildings;
• The development of a sprayed concrete radial joint, without connecting reinforcement, to eliminate the need of workers entering the face;
• Optimised SCL lining thicknesses to reduce the risk of fallouts and improve construction cycles;
• Change of proposed hand mining operations to sprayed concrete linings for improved workability;
• Application of 3D ground modelling; and
• Adoption of a risk-based approach to design and construction.

In early 2017, with the primary design stage complete, work began on site with construction of the main access shaft at Arthur Street. Sheet piling supported the upper part of the 33m deep x 14.6m long x 9.5m wide shaft with SCL taking the excavations to depth. Working closely with Cemex and Normet, a shotcrete mix design was developed and verification trials were carried out to ensure compliance with specified performance criteria. The shotcrete dry mix was supplied by Cemex and stored in four 100 tonne silos on the Arthur Street worksite.

View from the bottom of the 33m deep main access shaft

SCL advances through London Clay by excavator

Cast iron lining of existing and live Northern line exposed

The shaft was situated within a few metres of the piles to an adjacent new building that was nearing completion and excavated through the existing disused King William Street Station, which was partially backfilled with foam concrete, and later used as a pedestrian access route into the new tunnels (Fig 2). Due to the confined nature of the 80m x 10m Arthur Street worksite, and with more than 200,000 tonne of spoil to be removed from the 4m x 8.5m opening at the top, it was important to optimise the site logistics. A 11m³ Bulk-X muck skip system from Conquip was utilised to remove the spoil from the shaft. Initially the shotcrete was pumped from the surface to the tunnel face. Beyond 150m, shotcrete was transported in remixers.

Concurrently, assessment of existing London Underground assets continued, and design of mitigation measures to be employed to strengthen vulnerable assets was finalised. Mitigation measures included grouting of any gaps discovered behind existing masonry walls and lintel strengthening around existing openings with steel plates and grout. No internal propping within existing assets were required.

By September 2017, construction of the Arthur Street shaft was complete together with excavation and temporary and permanent linings of a plant chamber, logistics chamber and muck chamber at the bottom of the shaft and breakouts from the shaft were also established. The muck chamber required excavation right up to the back of the cast iron rings of the existing, live Northern line running tunnel. Through a combination of careful excavation and detailed monitoring this was completed successfully, with no impact to the train operations.

It was these initial tunnelling works that saw the development of what became known as the Option A SCL radial joint. Designed by Dr Sauer & Partners this sprayed radial joint detail between the SCL top heading and bench excavations eliminates the requirement for the installation of reinforcement bars (Fig 3). The joint is formed fully using mechanised means, eliminating the need for the workforce to access close to the face and avoiding the need for a temporary SCL sealing layer of the face. The stepped SCL lining application used the robotic concrete spraying equipment with the excavator breaking out the SCL lining toe for the joint preparation and cleaning. This innovation is a significant advancement for the SCL tunnelling industry by making the works considerably safer.

Fig 3. Design of a fully mechanised joint between top heading and bench eliminating need of workers in the face

After the completion of the logistic support chamber, works began to extend the new southbound Northern line running tunnel to the northwest. Full-face excavation on the 19m² heading on 1.2m advances resulted in efficient progress for work to begin early on the pilot heading of the platform tunnel, which was excavated as a mitigation measure to minimise potential surface settlement. By February 2018 crews had completed the 120m long x 19m² pilot tunnel and the subsequent enlargement to the full platform cross section of 10.5m wide at spring line x 9.6m high.

Load transfer of piles

During excavation of the new running tunnel, existing foundation piles were encountered as expected and had to be managed. A thorough desktop study during the design phase, coupled with 3D modelling, accurately located all the piles that were expected to be encountered. This detailed work was supported by collaborative discussions between Dragados and London Underground and the affected building owners and their engineering representatives. A thorough building damage assessment and extensive assessments of the building foundations were required to ensure structural integrity of the buildings. Four 20m deep x 1.8m diameter end bearing foundation piles supporting an existing 1970s office building also required interception (Figs 4 and 5).

Interception included design of a load transfer structure which bear the load of each cut foundation pile and transfers the load of up to 10MN each to an arched reinforced concrete structure built around the cut end of each intercepted pile. The structure bridges over the new Northern line southbound tunnel and delivers the loads to a reinforced concrete invert (Fig 6). The loads are then diffused to the ground. For the 1970s office building, the load transfer structures were designed for higher loads to support potential future storeys to the building.

In just under five months and with a little more than 600m³ of concrete and 35 tonnne of steel, the transfer structures were complete. Through a combination of careful excavation, detailed monitoring and full understanding and appreciation of the design by all involved in the construction, the transfer structures reached successful completion in late November 2018.

Cross passage excavation

To construct three cross passages connecting the operational DLR platform and concourse tunnels, which were below the level of the new Northern line tunnels, a hybrid solution of SCL shafts and access canopies, coupled with traditional square works excavation techniques and steel temporary frames, was utilised. This hybrid solution of SCL and steel support frames was further utilised to construct a 12m long and 10m deep lift shaft and stair core between the operational DLR tunnels which provided significant cost, time and health and safety benefit compared with a traditional squareworks technique.

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  Fig 4. Plan of a set of piles to be intercepted

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  Fig 5. CAD modelling for intercepting foundation piles

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  Fig 6. Transfer of load through the transfer structure to the ground

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  Fig 7. Exposed pile prior to being cut through

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  Dense cage of reinforcement in the transfer structure crown

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  Finalised transfer structures

Another challenging set of work was construction of three 4m wide adits connecting the existing and operational Northern line platform tunnels, which are just 900mm apart. The preliminary design was to construct hand-mined timber headings over the Northern line platform tunnels to gain access to construct the adits. A full redesign of the temporary squareworks was carried out using 3D FEM to refine the construction sequence and change the design to a sprayed concrete lined access tunnel and SCL canopy spanning the two platform tunnels. This eliminated the need for small timber box headings and significant amounts of hand mining, which greatly improved the excavation process for the crews. During excavation, the operating platform tunnels were kept operational and no propping was required inside the cast iron segmental lining. Movements to both platform structures during adit excavation were as predicted and below 40mm on the diameter.

To limit impact on the platform tunnels, a system of exterior walers and struts, which did not need to be bolted to the exterior of the cast iron segments, was designed and installed as excavation progressed. The lean squareworks ground support system also maximised the space between the back of the platform linings. At the springline the excavated space was less than 1m wide.

By late Feb 2019, and two years after starting the central access shaft, 1km of new tunnels had been excavated, creating space through a dense maze of existing and operational tunnels, building foundations and historic assets.

By July 2019, construction of the new SCL structures were 90% complete and focus shifted to the escalator tunnels connecting the Northern line with the Central line and the DLR, as well as with the new ticket hall at Cannon Street. The escalators represented a significant challenge, due in part to their steep gradient and the numerous existing assets in their way.

Escalator barrel excavations

Escalator 2 is a 7.7m x 6.4m high x 30 degrees incline between the Northern line level and the surface. Work started with the breakout from a piled box which formed part of the basement of the new ticket hall structure. Excavation then progressed top down in a sequence of top headings and combined bench/inverts. A walking excavator, commonly called a spider excavator, was used for excavation on the incline.

The escalator barrel connected into a previously constructed SCL cross passage located at 20m below at the Northern Line platform level. All excavators and spraying machines were hooked to a winch situated within the piled box at the top of the escalator barrel, and all personnel wore harnesses connected to inertia reels at all times, to ensure health and safety.

Design and application of the Option A sprayed radial joint was especially beneficial on the inclined escalator tunnels. After SCL primary lining completion, the works continued with the application of the spray applied waterproof membrane. Dragados worked closely with Mapei who provided a dustless spray applied waterproof membrane, which greatly improved the quality of the works while completely eliminating the dust during the application, thereby improving safety on site. This initiative was awarded a Safety Initiative of the Year Award in 2019.

Unavoidable uphill excavation of escalator 4

Escalator 4, a 8.2m wide x 6m high x 30 degree inclined tunnel, posed a greater technical challenge with its construction requiring uphill excavation. This was necessary due to its location between the existing and operational Central line platform tunnel assets and with no access for tunnel plant and material from the top. Existing shafts and passages present between the two Central line platform tunnels were foam concrete backfilled. A bespoke SCL excavation and support sequence was developed to facilitate tunnelling through these backfilled structures including the removal of cast iron lining segments and internal steel and concrete support elements. The sequence further considered the possibility of pre-disturbed ground conditions around them from their construction in the past and aspects of health and safety in a complex work environment.

One of the most complex structures of the works was construction of the Central line escalator which, due to its location underneath the existing station ticket hall and with five operational tunnels within 2.5m, had to be excavated uphill. It was constructed also under the historic 1752 Mansion House building. The structure was constructed in three parts, each with their own challenges.

First, an 8.5m diameter chamber was constructed perpendicular from a 5m diameter tunnel. This required a temporary tunnel to be constructed, with each advance requiring a different geometry, followed by back-mining to enlarge the chamber to the full profile. Back-mining is difficult to carry out and the risks were carefully managed through a bespoke design sequence for excavation and support as well as through detailed safe systems of work from Dragados.

Installation of waterproofing membranes

Uphill excavation of the 8m diameter escalator barrel at a 30-degree incline also posed particular risks and is only considered as a last resort. Close collaboration between design and construction was required to complete excavation successfully.

At the top of the incline a 9.5m high upper machine chamber was mined through two existing 100-year-old cast-iron shafts and two cast-iron tunnels, all backfilled with foam concrete. The excavation was squeezed between the two operational Central line platforms, with passengers just 600mm from the lining.

The construction of escalator 4 posed unprecedented risks and pushed the limitations of both tunnelling equipment on the market and the use of the SCL technique. Dragados worked closely with specialist equipment manufacturers to source the equipment for the complex works, which in the end required five different types of excavators and four different types of SCL spraying robots. Special care was required to ensure the stability of the machines while working on the steep inclines. Due to the complexity of the works and the high risks involved, the workforce needed to clearly understand the methodology. Each of the 86 excavation advances were drawn as graphical images to create a storyboard briefing of the planned works for the workers.

Underground spaces created by SCL ready for construction of the inner architectural and M&E finishes

Following these works and in the remaining months of 2019 and into 2020, excavation continued to complete various cross passages at Northern line level, escalator 3 down to the DLR level, followed by a new DLR concourse and cross passages as well as the lower PRM (persons with reduced mobility) shaft at the DLR platform level. Following primary supported excavation the underground structures were finished with a waterproofing membrane and steel fibre reinforced sprayed or cast concrete secondary linings.

In July 2020, after approximately 1,200m of SCL excavation, 1,840 advances, 1,195 daily review meetings and 995 required excavation and support (RES) sheets, the SCL excavation works for the Bank Station upgrade were successfully completed with connections from the new underground structures into the existing underground station infrastructure.

The project delivered great moments for those involved and the result to date is state-of-the-art structures built for many generations to come. The collaborative work approach taken throughout planning continued during construction with close daily interaction between contractor, client and designer fostering a best-for-Bank project mentality. The works were chronicled in a television documentary to celebrate the achievement.